Multilayer semiconductor structures or wafers are formed by transfer of at least one layer onto a final substrate. Such a layer transfer is obtained by bonding, for example, by molecular adhesion, of a first wafer (or initial substrate) onto a second wafer (or final substrate), the first wafer generally being thinned after bonding. The transferred layer may furthermore comprise all or part of a component or of a plurality of micro-components.
More precisely, this disclosure relates to the problem of the bonding defects that may occur in a localized manner at the bonding interface between two wafers bonded by molecular adhesion.
Bonding by molecular adhesion is a technique well known per se. As a reminder, the principle of bonding by molecular adhesion is based on bringing two surfaces into direct contact, in other words, without the use of a specific material (glue, wax, brazing, etc.). Such an operation requires the bonding surfaces to be sufficiently smooth, free from particulates or from contamination, and that they are sufficiently close together to allow a contact to be initiated, typically at a distance of less than a few nanometers. In this case, the forces of attraction between the two surfaces are sufficiently high to cause molecular adhesion (bonding induced by all the electronic interaction forces of attraction (van der Waals forces) between atoms or molecules of the two surfaces to be bonded).
FIGS. 1A to 1D show one exemplary embodiment of a multilayer structure comprising bonding by molecular adhesion of a first wafer 102 onto a second wafer 106, the latter forming a support wafer.
The first wafer 102 here comprises a series of micro-components 104 on its bonding face 102a (FIG. 1A). The micro-components 104 are formed by photolithography by means of a mask allowing areas for formation of patterns to be defined that correspond to the micro-components 104 to be formed.
In this document, the term “micro-components” is understood to mean the devices or any other patterns resulting from the technological steps carried out on or within the layers and whose positioning must be controlled with precision. These can, therefore, be active or passive components, simple contacts, interconnections, etc.
In this example, the support wafer 106 is covered by a layer of thermal oxide 108 (or deposited oxide) formed, for example, by oxidation of the support wafer 106 in order to facilitate the molecular adhesion with the first wafer 102 (FIG. 1A).
Some form of treatment is generally implemented in order to prepare the bonding surface 102a of the first wafer 102 and the bonding surface 106a of the second wafer 106, this treatment varying according to the bonding energy that it is desired to obtain (chemical-mechanical polishing (CMP), cleaning, brushing, hydrophobic/hydrophilic treatment, etc.).
Once the wafers 102 and 106 have been prepared, the support wafer 106 is positioned in a bonding machine 115. More precisely, the support wafer 106 is positioned on a substrate holder 110 of the bonding machine 115 with a view to its assembly by molecular adhesion with the first wafer 102. The substrate holder 110 holds the second wafer 106 in position by means, for example, of an electrostatic or suction system.
The first wafer 102 is subsequently placed onto the second wafer 106 so as to be in intimate contact with the latter (FIG. 1B). The initiation of the molecular adhesion is then carried out by application of a contact force (mechanical pressure) on the first wafer 102 (FIG. 1C). The application of this contact force allows the propagation of a bonding wave 122 to be initiated starting from this point of initiation (FIG. 1D). The bonding wave 122 is initiated by means of an application tool 114 (a TEFLON® stylus, for example) with which the bonding machine 115 is equipped.
In this document, “bonding wave” refers to the molecular bonding or adhesion wavefront that propagates from the point of initiation and corresponds to the diffusion of the attractive forces (van der Waals forces) from the point of contact over the whole intimate contact surface (bonding interface) between the two wafers.
The propagation of the bonding wave 122 over the entirety of the bonding surfaces of the wafers 102 and 106 thus allows the bonding by molecular adhesion of the two wafers, so as to obtain a multilayer structure 112.
Once the bonding has been effected, the bonding may be reinforced by implementing a thermal annealing. The first wafer 102 may subsequently be thinned in order to form a layer transferred onto the support wafer 106.
The applicant has, however, observed the presence of localized bonding defects 118 at the bonding interface between the two wafers 102 and 106, and more precisely in a region 120 situated at the opposite side from the bonding initiation point 116 (FIG. 1E). These defects correspond to regions in which the two wafers 102 and 106 exhibit a very low bonding force, or even a total absence of bonding.
These bonding defects are undesirable for the manufacturer since they reduce the quality of the bonding between the wafers. More generally, these defects are an indication of a non-optimized fabrication process, a fact that reduces the attractiveness of the multilayer structures thus produced.
It is currently, therefore, necessary to improve the quality of fabrication of the multilayer structures assembled by molecular adhesion. In particular, there exists a need for a bonding process using molecular adhesion allowing the appearance of the aforementioned bonding defects at the bonding interface between the wafers to be reduced, or even completely prevented.